Introduce CoroutineDispatcher.asExecutor() extension

Fixes #1450

Author: elizarov

binary-compatibility-validator/reference-public-api/kotlinx-coroutines-core.txt

@@ -293,6 +293,7 @@ public abstract class kotlinx/coroutines/ExecutorCoroutineDispatcher : kotlinx/c
 }
 
 public final class kotlinx/coroutines/ExecutorsKt {
+	public static final fun asExecutor (Lkotlinx/coroutines/CoroutineDispatcher;)Ljava/util/concurrent/Executor;
 	public static final fun from (Ljava/util/concurrent/Executor;)Lkotlinx/coroutines/CoroutineDispatcher;
 	public static final fun from (Ljava/util/concurrent/ExecutorService;)Lkotlinx/coroutines/ExecutorCoroutineDispatcher;
 }

kotlinx-coroutines-core/jvm/src/Executors.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.coroutines
@@ -35,15 +35,27 @@ public abstract class ExecutorCoroutineDispatcher: CoroutineDispatcher(), Closea
  */
 @JvmName("from") // this is for a nice Java API, see issue #255
 public fun ExecutorService.asCoroutineDispatcher(): ExecutorCoroutineDispatcher =
-    // we know that an implementation of Executor.asCoroutineDispatcher actually returns a closeable one
-    (this as Executor).asCoroutineDispatcher() as ExecutorCoroutineDispatcher
+    ExecutorCoroutineDispatcherImpl(this)
 
 /**
  * Converts an instance of [Executor] to an implementation of [CoroutineDispatcher].
  */
 @JvmName("from") // this is for a nice Java API, see issue #255
 public fun Executor.asCoroutineDispatcher(): CoroutineDispatcher =
-    ExecutorCoroutineDispatcherImpl(this)
+    (this as? DispatcherExecutor)?.dispatcher ?: ExecutorCoroutineDispatcherImpl(this)
+
+/**
+ * Converts an instance of [CoroutineDispatcher] to an implementation of [Executor].
+ *
+ * It returns an original executor when used on the result of [Executor.asCoroutineDispatcher] extensions.
+ */
+public fun CoroutineDispatcher.asExecutor(): Executor =
+    (this as? ExecutorCoroutineDispatcher)?.executor ?: DispatcherExecutor(this)
+
+private class DispatcherExecutor(@JvmField val dispatcher: CoroutineDispatcher) : Executor {
+    override fun execute(block: Runnable) = dispatcher.dispatch(EmptyCoroutineContext, block)
+    override fun toString(): String = dispatcher.toString()
+}
 
 private class ExecutorCoroutineDispatcherImpl(override val executor: Executor) : ExecutorCoroutineDispatcherBase() {
     init {

kotlinx-coroutines-core/jvm/test/ExecutorsTest.kt

@@ -1,11 +1,13 @@
 /*
- * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.coroutines
 
-import org.junit.Test
-import java.util.concurrent.Executors
+import org.junit.*
+import org.junit.Assert.*
+import java.util.concurrent.*
+import kotlin.coroutines.*
 
 class ExecutorsTest : TestBase() {
     private fun checkThreadName(prefix: String) {
@@ -32,14 +34,49 @@ class ExecutorsTest : TestBase() {
     }
 
     @Test
-    fun testToExecutor() {
+    fun testExecutorToDispatcher() {
         val executor = Executors.newSingleThreadExecutor { r -> Thread(r, "TestExecutor") }
         runBlocking(executor.asCoroutineDispatcher()) {
             checkThreadName("TestExecutor")
         }
         executor.shutdown()
     }
 
+    @Test
+    fun testConvertedDispatcherToExecutor() {
+        val executor: ExecutorService = Executors.newSingleThreadExecutor { r -> Thread(r, "TestExecutor") }
+        val dispatcher: CoroutineDispatcher = executor.asCoroutineDispatcher()
+        assertSame(executor, dispatcher.asExecutor())
+        executor.shutdown()
+    }
+
+    @Test
+    fun testDefaultDispatcherToExecutor() {
+        val latch = CountDownLatch(2)
+        Dispatchers.Default.asExecutor().execute {
+            checkThreadName("DefaultDispatcher")
+            latch.countDown()
+        }
+        latch.countDown()
+    }
+
+    @Test
+    fun testCustomDispatcherToExecutor() {
+        expect(1)
+        val dispatcher = object : CoroutineDispatcher() {
+            override fun dispatch(context: CoroutineContext, block: Runnable) {
+                expect(2)
+                block.run()
+            }
+        }
+        val executor = dispatcher.asExecutor()
+        assertSame(dispatcher, executor.asCoroutineDispatcher())
+        executor.execute {
+            expect(3)
+        }
+        finish(4)
+    }
+
     @Test
     fun testTwoThreads() {
         val ctx1 = newSingleThreadContext("Ctx1")